The invention relates to a circuit arrangement for protecting against switching overvoltages power semiconductor switches which can be switched on and off via their control input and form a pair of arms in a power converter.
During switching operations in power converters, inductances in the power section of power converters cause overvoltages which can lead to damage to the power semiconductors, depending on their level and duration. Above all, the relatively quickly switching insulated gate bipolar transistor (IGBT) modules are themselves endangered in this case. In order to protect against such overvoltages, it is advantageous firstly to minimize the parasitic inductances in the main circuit by means of favorable conductor routing. Various wiring networks are then used and connected on the shortest path. These networks have the task of guaranteeing the operation in the permitted operating range, as well as occasionally reducing the switch-off power loss. An individual RCD circuit has been used for many years to protect power semiconductors and is also suitable for IGBT modules, for example. The arrangement consists of a capacitor which is connected in series with a diode with a parallel resistor. If, for example, at a relatively high pulse frequency the power loss converted into heat at the resistor assumes substantial values, this is something which is fundamentally undesirable. More frequently, use is made of more cost-effective measures (compare etz, Volume 110 (1989), pages 464-471) such as RCD voltage limiters for pairs of arms (FIG. 6b) or the summation circuit on the DC side (FIG. 6d), which moreover cause less power loss. In return, however, the latter are also not quite so effective. RCD voltage-limiting circuits are frequently also referred to as voltage-clamping circuits.
The aim of voltage-clamping circuits is to limit the overvoltage which is produced upon switching power semiconductor switches (denoted by PSC below) owing to inductances which are mostly determined by design, to a value which is safe for the PSC.
If only the known (RCD) wiring network is used as "voltage-clamping circuit" for the purpose of voltage limitation, it is to be dimensioned for the highest possible switch-off current. If, however, it is also desired to utilize the ability of modern PSCs, which can even switch off short-circuit currents which can reach more than ten times the periodically permitted current, the circuit capacitor is to be of an appropriately large dimension for the short-circuit case. It must be taken into account on so doing that the energy stored in the design inductor increases with the square of the switch-off current. A large circuit not only means a larger outlay on components and thus higher costs, but, moreover, there is also an increase in losses produced in the RCD circuit. In the case of high switching frequencies (f.sub.s =10 kHz), relatively large power losses are to be dissipated via the circuit resistor. Not only does this complicate the design technology, there is also an increase in the outlay on cooling; and the efficiency of the circuit drops not inconsiderably.
If use is made of a voltage-clamping circuit by means of a zener diode between a main terminal (collector, drain) and the control terminal (gate, base) of a power MOSFET (Bull. ASE/UCS 77 (1986) 16, 23rd August, pages B388-B391), the zener voltage of the zener diode is dimensioned at the maximum permissible peak voltage in the circuit.
If the voltage peak caused by the design inductor is higher than the zener voltage, a (low) current can flow via the zener diode to the control terminal of the power semiconductor switch (for example power MOSFET, IGBT) and turn the latter on. However, this happens ideally only to a certain degree, with the result that the power semiconductor switch is driven just so far that the voltage across a main path (C-E or D-S or C-S, depending on component type) continues to correspond exactly to the zener voltage until the flow of current in the main path is terminated and the design inductor has completely given up the stored energy to the semiconductor switch. A voltage-clamping circuit operating by means of a zener diode between a main terminal and the control terminal of a power semiconductor switch has the following disadvantages:
--owing to the manufacturing tolerances and the temperature coefficient of the zener diodes, the voltage limitation is very imprecise (approximately .+-.5% manufacture plus approximately 10% additional error for a temperature range of 100 K.), PA1 --the power loss of the zener diode is so large that the function of this voltage limitation can generally be used only for rare cases of fault and not at the envisaged switching rate in continuous pulsed operation, and therefore PA1 --there is a need for additional circuits of the power transistors in order to limit the voltages during operation, which are known to cause a large and undesired power loss in the circuit resistors (loss-free wiring networks require an even higher outlay).